1. Technical Field
Optical spectrum separation systems used in creating separate images from different wavelength bands of light; and methods for distinguishing different sources of illumination based on differences in the optical spectrum.
2. Description of Related Art
In many fields it is desirable to identify the potential sources of different types of visible or invisible illumination captured by an imaging system, such as by a digital camera. An example is differentiating between “muzzle flash” from a weapon and bright reflections caused by sunlight. Often the spectral signature of two events will be quite different, for example at infrared wavelengths. Therefore, one way to differentiate potential sources of illumination is to separately image different portions of the spectrum and then compare the two images.
One method to separate incoming illumination into separate spectral components is to use dichroic beam splitters. For example, U.S. Pat. No. 4,806,750, the disclosure of which is incorporated herein by reference, discloses a trichromatic beamsplitter comprising composited dichroic beamsplitter plates that separate a projected image into its three color components with spatial as well as spectral precision. The three colors are directed toward a photosensor with three linear arrays. The three linear array photosensor comprises a monolithic sensor having three parallel photodiode arrays spaced precisely to accept the color component images of the trichromatic beamsplitter. Because this invention utilizes linear arrays, and it is intended for use in scanning systems, it is generally not suitable for simultaneously distinguishing transient illumination sources at different spatial locations.
U.S. Pat. No. 5,920,347, the disclosure of which is incorporated herein by reference, describes an optical color separation system that includes a plurality of dichroic mirrors that are arranged nonparallel to one another and along an optical axis of the incident light to reflect different color components. The dichroic mirrors are disposed at predetermined positions so that the respective color components of the incident light are reflected toward different relay positions corresponding to the respective color components. The optical color separation system also includes a plurality of reflection mirrors corresponding to the dichroic mirrors, provided at the relay positions. The reflection mirrors are disposed at predetermined positions such that the different color components reflected by the dichroic mirrors are reflected by the reflection mirrors in a manner that optical paths of the reflected lights from the respective reflection mirrors are substantially parallel.
An alternative method to create separate spectral images is to first split the incoming light using neutral-density beam splitters, and then to filter the spectrum optically at the focal plane. For example, U.S. Pat. No. 5,621,460, the disclosure of which is incorporated herein by reference, describes an optical system for detecting vegetation using a single image sensor, such as a charge-coupled device (CCD). The system is comprised of optical elements which project separate red and near infrared (NIR) images onto the CCD camera. The camera can supply output signals representative of the two images to a signal processing device that determines the nature of the vegetation that provided the original image. Embodiments are disclosed wherein the imaging sensor utilizes unitary optical splitters to provide wide-angle separate images. A further embodiment includes two separate lens, red and NIR filters, each adjacent to one of the lenses so as to project an image on separate areas of the CCD camera.
The above disclosures and others in the art notwithstanding, there remains a need for an optical spectrum separation system and methods for creating separate images from different wavelength bands of light that is of low complexity, and that is adaptable to two dimensional still or video imaging.